1. Field of the Invention
The invention relates to electrical terminals for resistive elements, and in particular, to an electrical supporting terminal for high temperature element, such as a molybdenum disilicide resistance element used as an igniter or sensor for a flame.
2. Description of the Prior Art
Prior art terminals for high temperature or igniter elements including molybdenum disilicide elements, as exemplified in U.S. Pat. Nos. 895,857, 1,496,569, 2,384,797, 3,307,136, 3,522,574, 3,562,590, 3,569,787 and 3,662,222, have been made by crimping, brazing, soldering, welding, bonding or otherwise joining metal terminal members or sleeves to elements. Commerically available molybdenum disilicide elements containing minor portions of ceramics or other materials initially have substantial strength and ability to withstand shock. However many prior art terminals for molybdenum disilicide elements have been of limited suitability due to failure at or near the junction between the element and the terminal members during the joining process, handling shocks or repeated use; some of the failures result from rapid deterioration in strength and conductivity of the materials used in making the terminals or junctions. Welding the elements to heat-resistant metal terminals avoids failures due to rapid deterioration in strength and conductivity of the terminals; however, welding of molybdenum disilicide elements to commonly used heat-resistant metals has previously not been entirely satisfactory because the elements have tended to become weakened and embrittled at or near the welded junction tending to break during or after the welding process. It has been previously suggested that this weakening and embrittlement of molybdenum disilicide elements is caused by relatively large temperature gradients. Also some failures of molybdenum disilicide elements in welded junctions to heat resistance metal terminals have been attributed to the result of dissimilar temperature expansion coefficients and other incompatable metal properties. Provision of separate mechanical support for molybdenum disilicide elements eliminates some of the failures during subsequent handling; however, such provision has not been completely successful, and also failures still occur prior to providing the mechanical support.
One significant breakthrough in the formation of terminals on refractory resistance elements has been the employment of a metal supporting member, such as a sleeve, extending along a segment of a refractory resistance element with one end of the resistance element and supporting member fused together and with the unfused portion of the supporting member extending along a substantial portion of the segment providing mechanical support for the element where subject to weakening. Also this prior art refractory resistance and terminal included a layer of malleable metal, such as silver, interposed between the unfused portion of the supporting member and the segment of the element; such malleable metal including a solidified-from-melt portion which was melted during the fusing of the supporting member and the refractory resistance element and then cooled to surround and engage a portion of the segment of the element. This prior art refractory resistance and terminal is shown in U.S. Pat. No. 3,969,696 granted July 13, 1976.
Additionally, a number of techniques have been developed in the prior art to connect dissimilar metal members, such as conductors, tubes, etc., used in relatively low temperature applications as exemplified in U.S. Pat. Nos. 2,914,641, 3,244,798 and 3,656,092. Generally, such techniques are inapplicable or unsuitable for high temperature resistance elements, such as molybdenum disilicide elements, in that the materials employed are substantially deteriorated in strength and conductivity by high temperatures, air and fuel.